1. Field of the Invention
The present invention relates to a coordinate reading device.
2. Description of Related Art
Various types of coordinate reading device have been proposed. Representative examples of the conventional coordinate reading device are disclosed in Japanese Patent Application Publication (Kokai) Nos. HEI-5-165560, HEI-2-190199, HEI-8-202491, and HEI-5-143224 and in Japanese Patent Application Publication (Kokoku) No. SHO-58-16507.
Japanese Patent Application Publication (Kokoku) No. SHO-58-16507 describes one type of the coordinate reading device. As shown in FIG. 1, the coordinate reading device of this type has a tablet provided with a sense loop (conductive wire) 391. The sense loop 391 is mounted on the tablet so as to extend following a rectangular-wave shape. The coordinate reading device detects the position of a pen or position indication unit that is mounted with a magnetic coil 390. A correction sense loop 392 is additionally provided around the sense loop 391. The sense loop 391 generates an induction signal due to magnetic coupling with a magnetic field generated at the magnetic coil 390. The induction signal, however, may receive influence from the peripheral portion of the sense loop 391. According to this coordinate reading device, such a portion of the induction signal that receives the influence is canceled out by the induction signal generated by the correction loop 392 so that an error in the positional coordinates of the pen at the periphery portion can be corrected.
Japanese Patent Application Publication (Kokai) No. HEI-2-190199 discloses a coordinate detecting device that calculates positional coordinates of a pen or position indication unit by obtaining the number of pulses obtained from the outputs of a conductive wire. When the indication device is positioned at an edge portion of the coordinate detecting device, the positional coordinates are corrected by adding or subtracting a predetermined pulse number from the obtained pulse numbers.
However, the coordinate reading device disclosed in Japanese Patent Application Publication (Kokai) No. SHO-58-16507 has a problem in that efficiency of manufacturing the tablet is low because extra operations are required to mount the correction loop 392 around the sense loop 391. Also, because the correction loop 392 has to be provided around the sense loop 391, the surface area of the coordinate input sheet where the correction loop 392 is provided cannot be used for detection, but is wasted in vain.
The coordinate retrieval device described in Japanese Patent Application Publication (Kokai) No. HEI-2-190919 has a problem in that circuitry for counting the number of pulses and for adding or subtracting pulse number has to be provided.
FIG. 2(a) is a configuration of a conceivable coordinate reading device. The conceivable coordinate reading device includes: a tablet 91, a scanning circuit 92, and a detection circuit 90. The tablet 91 is provided with a plurality of X sense coils (conductive wires) X1 to Xm and Y sense coils (conductive wires) Y1 to Yn. The X sense coils X1 to Xm are for detecting X coordinates of a pen 400, the Y sense coils Y1 to Yn are for detecting Y coordinates of the pen 400. The scan circuit 92 is for serially scanning the sense coils X1-Xm and Y1-Yn on the tablet 91. The detection circuit 90 is for calculating the x and y coordinates of the pen by detecting induction signals generated at the sense coils X1-Xm and Y1-Yn.
The pen or position indication unit 400 includes a coil 401 that generates an alternating magnetic field. When the pen 400 contacts the tablet 91, some sense coils that are located near to the pen 400 generate induction signals 97 due to magnetic coupling with the alternating magnetic field from the coil 401. The induction signals 97 are inputted into the detection circuit 90. The induction signals 97 are amplified at an amplifier 93 and then its amplitude is detected at an amplitude detection circuit 94. Next, an A/D conversion circuit 95 converts the measured amplitude into digital values, and outputs the digital values to a CPU 96. The CPU 96 calculates the positional coordinate of the pen 400 based on the inputted digital values from the A/D conversion circuit 95. For example, the CPU 96 may refer to a table that stores a relationship between a plurality of digital values and coordinate positions. The CPU 96 selects a positional coordinate that corresponds to the digital values presently inputted from the A/D conversion circuit 95.
FIG. 2(b) is a graph showing the relationship how the voltage generated at the sense coil Y1 changes when the position of the electric pen 400, defined as the distance D from the edge Y1d of the sense coil Y1, changes. As shown in FIG. 2(b), when the pen 400 is located at a position P1 in the sense coil Y1, the sense coil Y1 generates voltage value V1. However, the closer the pen 100 is to the lengthwise end (short side Y1d) of the sense coil Y1, the generated voltage value V is effected by the induction signal from the short side Y1d. The voltage V rises, and the voltage value increases from V1 to V2 when the pen 400 is at position P2. The closer the pen 400 is to the short side Y1d from the position P2, however, the voltage value V drops.
As can be seen, in this conceivable technology, the voltage detected by the detection circuit 90 changes as the pen goes near to the edges of the tablet 91. It is impossible to accurately detect the position of the pen when the pen is located near to the edges of the tablet 91. The range of the tablet that can be used for accurate detection is narrower than it is desired.
Japanese Patent-Application Publication No. HEI-8-202491 discloses decreasing the width of the sense coils at their edge portions. Japanese Patent-Application Publication No. HEI-5-143224 discloses using a sub loop. However, with both of these methods, the configuration at the edge portions becomes quite complicated, or the number of coils need to be increased, which also complicates the configuration.
Additionally, in the conceivable technology of FIG. 2(a), the voltage detected by the detection circuit 90 increases as the pen goes near to the edges of the tablet 91. When the pen 400 is located near to some edge of the tablet 91, therefore, the digital amplitude value outputted from the A/D conversion circuit 95 will increase. Therefore, the burden on the CPU 96 increases. Also, the dynamic range of the amplifier 93 has to be broadened to match the voltage values at the edge portions of the tablet 91.
In view of the above-described drawbacks, it is an objective of the present invention to provide an improved coordinate reading device that has a simple configuration but that is capable of accurately reading coordinates of a target (alternating magnetic field generating unit), even when the target is located at some edge portion of the coordinate reading device.
In order to attain the above and other objects, the present invention provides a coordinate reading device reading a coordinate of a position of a target which generates an alternating magnetic field, the coordinate reading device comprising: a base member; a plurality of loop wires provided on the base member, each loop wire being capable of producing an electric signal in response to an alternating magnetic field generated from a target, each loop wire having at least one edge portion; a detection unit that detects levels of the signals generated from the plurality of loop wires, thereby determining a coordinate of the position of the target; and a correction unit correcting the coordinate of the position of the target when the target is located in each of the at least one edge portion of some loop wire.
The correction unit may correct each loop wire to produce a corrected signal when the target is located in each of the at least one edge portion of some loop wire. The base member may have a front surface, a rear surface, a pair of opposite side surfaces, each of which connects the front surface and the rear surface, a pair of opposite edges being defined between the front surface and the pair of side surfaces, the target being located on the side of the front surface. Each loop wire may be provided on the front surface, and has a pair of first sides that extend on the front surface and a pair of second sides connecting the pair of first sides therebetween, a pair of edge portions corresponding to the pair of second sides. In this case, the correction unit may provide each of the pair of second sides on either one of the corresponding side surface and the rear surface by bending the first side at the corresponding edge of the base member in a direction toward the rear surface.
The correction unit may correct the detection unit to determine a corrected coordinate of the target. The plurality of loop wires may include a plurality of first loop wires and a plurality of second loop wires, the plurality of first loop wires being arranged in a predetermined first direction with a predetermined pitch, the plurality of second loop wires being arranged in a predetermined second direction with the predetermined pitch, the plurality of first loop wires having a pair of edge portions along the second direction, the plurality of second loop wires having a pair of edge portions along the first direction. The detection unit may include: a signal level detection unit that detects the levels of the signals produced by the first and second loop wires; and a coordinate determination unit that is capable of determining the coordinates, along the first and second directions, of the position of the target based on the detected signal levels. In this case, the correction unit may include: judging means that judges, based on the determined coordinate along either one of the first and second directions, whether the target is within one of the pair of edge portions in the other one of the first and second loop wires; and coordinate correction unit that corrects, when the target member is judged to be located in one edge portion of the other one of the first and second loop wires, the coordinate of the target along the other one of the first and second directions using a correction value whose value corresponds to the coordinate of the target along the either one of the first and second directions.